Control of conductivity in anaerobic fermentation

ABSTRACT

Process are provided which are effective for controlling medium conductivity during fermentation of a CO-containing gaseous substrate while providing an STY of about 10 g ethanol/(L·day) or more. The process includes balancing medium conductivity, specific carbon uptake or cell density levels.

This application claims the benefit of U.S. Provisional Application No.61/833,189 which was filed on Jun. 10, 2013, and which is incorporatedin its entirety herein by reference.

A process is provided for controlling conductivity during syngasfermentation and maintaining an STY of about 10 g ethanol/(L·day) ormore. More specifically, processes for controlling conductivity includebalancing medium conductivity, specific carbon uptake, or cell density.

BACKGROUND

Anaerobic microorganisms can produce ethanol from CO throughfermentation of gaseous substrates. Fermentations using anaerobicmicroorganisms from the genus Clostridium produce ethanol and otheruseful products. For example, U.S. Pat. No. 5,173,429 describesClostridium ljungdahlii ATCC No. 49587, an anaerobic acetogenicmicroorganism that produces ethanol and acetate from synthesis gas. U.S.Pat. No. 5,807,722 describes a method and apparatus for converting wastegases into organic acids and alcohols using Clostridium ljungdahlii ATCCNo. 55380. U.S. Pat. No. 6,136,577 describes a method and apparatus forconverting waste gases into ethanol using Clostridium ljungdahlii ATCCNo. 55988 and 55989.

Acetogenic bacteria require a constant feed of nutrients for stableperformance and ethanol productivity. Higher productivity levels mayrequire the use of more concentrated mediums to provide effectiveamounts of nutrients. Use of more concentrated mediums results in afermentation broth with a higher ionic strength. Higher ionic strengthcauses detrimental effects on culture performance.

SUMMARY

Process are provided which are effective for controlling mediumconductivity during fermentation of a CO-containing gaseous substratewhile providing an STY of about 10 g ethanol/(L·day) or more. Theprocess includes balancing medium conductivity, specific carbon uptakeor cell density levels.

A process for fermenting a CO-containing gaseous substrate includesproviding a CO-containing gaseous substrate to a fermentation medium. Inone aspect, the process includes maintaining a conductivity to specificcarbon uptake (SCU in mmole/minute/gram dry cells) relationshipaccording to a formula where SCU=SCU_(max)−F*conductivity, whereinSCU_(max)=0 to 3 and F=0 to 1. The fermentation medium has aconductivity of about 30 mS/cm or less and the process is effective formaintaining an STY of about 10 g ethanol/(L·day) or more.

A process for fermenting a CO-containing gaseous substrate includesproviding a CO-containing gaseous substrate to a fermentation medium andfermenting the syngas. The process further includes maintaining aconductivity (y) to specific gas feed rate (x) according to a formulawhere y=−6.0327x+12.901, until reaching a target cell density, wherein xis about 0.2 to about 0.7 mmole/minute/gram of cells. In another aspect,the process includes maintaining a cell density above a target celldensity and maintaining a conductivity of about 30 mS/cm or less. Theprocess is effective for maintaining an STY of about 10 gethanol/(L·day) or more.

A process for fermentation of a CO-containing gaseous substrate includesintroducing the CO-containing gaseous substrate into a reactor vesselthat includes a fermentation medium and fermenting the CO-containinggaseous substrate. In one aspect of the process, at least one or morechloride ions in the fermentation medium are substituted with an ionselected from the group consisting of hydroxide, acetate, carbonate,bicarbonate and mixtures thereof in an amount effective for providing aconductivity of about 30 mS/cm or less. The process is effective formaintaining an STY of about 10 g ethanol/(L·day) or more.

BRIEF DESCRIPTION OF FIGURES

The above and other aspects, features and advantages of several aspectsof the process will be more apparent from the following figures.

FIG. 1 illustrates growth of Clostridium ljungdahlii in 1× growth mediumand a 25 ml/min syngas feed rate.

FIG. 2 illustrates growth of Clostridium ljungdahlii in 1× growth mediumand a 35 ml/min syngas feed rate.

FIG. 3 illustrates growth of Clostridium ljungdahlii in 1× growth mediumand a 40 ml/min syngas feed rate.

FIG. 4 illustrates growth of Clostridium ljungdahlii in 1× growth mediumand a 45 ml/min syngas feed rate.

FIG. 5 illustrates growth of Clostridium ljungdahlii in 1× growth mediumand a 50 ml/min syngas feed rate.

FIG. 6 illustrates growth of Clostridium ljungdahlii in 1× growth mediumand a 50 ml/min syngas feed rate with a higher initial inoculum.

FIG. 7 illustrates growth of Clostridium ljungdahlii in 1.5× growthmedium and a 45 ml/min syngas feed rate with a higher initial inoculum.

FIG. 8 illustrates growth of Clostridium ljungdahlii in 1.5× growthmedium and a 35 ml/min syngas feed rate with a higher initial inoculum.

FIG. 9 illustrates growth of Clostridium ljungdahlii in 1.5× growthmedium and a 30 ml/min syngas feed rate with a higher initial inoculum.

FIG. 10 illustrates growth of Clostridium ljungdahlii in 1.5× growthmedium and a 20 ml/min syngas feed rate with a higher initial inoculum.

FIG. 11 shows specific carbon uptake of Clostridium ljungdahlii growingin medium containing ammonium chloride.

FIG. 12 shows specific ethanol productivity of Clostridium ljungdahliigrowing in medium containing ammonium chloride.

FIG. 13 illustrates specific carbon uptake of Clostridium ljungdahliigrowing in medium containing 1-Lysine.

FIG. 14 illustrates specific ethanol productivity of Clostridiumljungdahlii growing in medium containing 1-Lysine.

FIG. 15 shows conductivity of Clostridium ljungdahlii growing in mediumcontaining 1-Lysine.

FIG. 16 shows specific carbon uptake of Clostridium ljungdahlii growingin medium containing ammonium acetate.

FIG. 17 illustrates specific ethanol productivity of Clostridiumljungdahlii growing in medium containing ammonium acetate.

FIG. 18 illustrates conductivity of Clostridium ljungdahlii growing inmedium containing ammonium acetate.

FIG. 19 shows specific carbon uptake of Clostridium ljungdahlii growingin medium containing ammonium carbonate.

FIG. 20 shows specific ethanol productivity of Clostridium ljungdahliigrowing in medium containing ammonium carbonate.

FIG. 21 illustrates conductivity of Clostridium ljungdahlii growing inmedium containing ammonium carbonate.

FIG. 22 illustrates specific carbon uptake of Clostridium ljungdahliigrowing in medium with ammonium carbonate as base.

FIG. 23 shows specific ethanol productivity of Clostridium ljungdahliigrowing in medium with ammonium carbonate as base.

FIG. 24 shows conductivity of Clostridium ljungdahlii growing in mediumwith ammonium carbonate as base.

Specific carbon uptake of Clostridium ljungdahlii growing in medium withammonium bicarbonate is shown in FIG. 25.

Specific ethanol productivity of Clostridium ljungdahlii growing inmedium with ammonium bicarbonate is shown in FIG. 26.

Conductivity of Clostridium ljungdahlii growing in medium with ammoniumbicarbonate is shown in FIG. 27.

FIG. 28 illustrates effects of step-wise increases in mediumconductivity on performance of Clostridium ljungdahlii.

FIG. 29 illustrates effects of step-wise increases in mediumconductivity on performance of Clostridium ljungdahlii.

FIG. 30 shows the relationship between specific CO feed rate andconductivity during fermentation of Clostridium ljungdahlii.

FIG. 31 shows the relationship between specific carbon uptake andconductivity during fermentation of Clostridium ljungdahlii.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the figures. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousaspects of the present process and apparatus. Also, common butwell-understood elements that are useful or necessary in commerciallyfeasible aspects are often not depicted in order to facilitate a lessobstructed view of these various aspects.

DETAILED DESCRIPTION

The following description is not to be taken in a limiting sense, but ismade merely for the purpose of describing the general principles ofexemplary embodiments. The scope of the invention should be determinedwith reference to the claims.

Syngas fermentations conducted in bioreactors with medium and acetogenicbacteria as described herein are effective for providing conversions ofCO in syngas into alcohols and other products. Control of conductivity,conductivity, and cell density is effective for providing highproductivity levels. In this aspect, alcohol productivity may beexpressed as STY (space time yield expressed as g ethanol/(L·day). Inthis aspect, the process is effective for providing a STY (space timeyield) of at least about 10 g ethanol/(L·day). Possible STY valuesinclude about 10 g ethanol/(L·day) to about 200 g ethanol/(L·day), inanother aspect, about 10 g ethanol/(L·day) to about 160 gethanol/(L·day), in another aspect, about 10 g ethanol/(L·day) to about120 g ethanol/(L·day), in another aspect, about 10 g ethanol/(L·day) toabout 80 g ethanol/(L·day), in another aspect, about 20 gethanol/(L·day) to about 140 g ethanol/(L·day), in another aspect, about20 g ethanol/(L·day) to about 100 g ethanol/(L·day), in another aspect,about 40 g ethanol/(L·day) to about 140 g ethanol/(L·day), and inanother aspect, about 40 g ethanol/(L·day) to about 100 gethanol/(L·day).

Definitions

Unless otherwise defined, the following terms as used throughout thisspecification for the present disclosure are defined as follows and caninclude either the singular or plural forms of definitions belowdefined:

The term “about” modifying any amount refers to the variation in thatamount encountered in real world conditions, e.g., in the lab, pilotplant, or production facility. For example, an amount of an ingredientor measurement employed in a mixture or quantity when modified by“about” includes the variation and degree of care typically employed inmeasuring in an experimental condition in production plant or lab. Forexample, the amount of a component of a product when modified by “about”includes the variation between batches in a multiple experiments in theplant or lab and the variation inherent in the analytical method.Whether or not modified by “about,” the amounts include equivalents tothose amounts. Any quantity stated herein and modified by “about” canalso be employed in the present disclosure as the amount not modified by“about”.

“Conductivity” and “average conductivity” refer to the ability toconduct electricity. Water conducts electricity because it containsdissolved solids that carry electrical charges. For example, chloride,nitrate, and sulfate carry negative charges, while sodium, magnesium,and calcium carry positive charges. These dissolved solids affect thewater's ability to conduct electricity. Conductivity is measured by aprobe, which applies voltage between two electrodes. The drop in voltageis used to measure the resistance of the water, which is then convertedto conductivity. Average conductivity may be measured by knowntechniques and methods. Some examples of average conductivitymeasurements are provided in ASTM D1125, “Standard Test Methods forElectrical Conductivity and Resistivity of Water”, and in “StandardMethods for the Examination of Water and Wastewater”, 1999, AmericanPublic Health Association, American Water Works Association, WaterEnvironment Federation, both of which are incorporated herein byreference.

The term “syngas” or “synthesis gas” means synthesis gas which is thename given to a gas mixture that contains varying amounts of carbonmonoxide and hydrogen. Examples of production methods include steamreforming of natural gas or hydrocarbons to produce hydrogen, thegasification of coal and in some types of waste-to-energy gasificationfacilities. The name comes from their use as intermediates in creatingsynthetic natural gas (SNG) and for producing ammonia or methanol.Syngas is combustible and is often used as a fuel source or as anintermediate for the production of other chemicals.

The terms “fermentation”, fermentation process” or “fermentationreaction” and the like are intended to encompass both the growth phaseand product biosynthesis phase of the process. In one aspect,fermentation refers to conversion of CO to alcohol.

The term “cell density” means mass of microorganism cells per unitvolume of fermentation broth, for example, grams/liter. In this aspect,the process and mediums are effective for providing a cell density of atleast about 1.0 g/L.

The term “cell recycle” refers to separation of microbial cells from afermentation broth and returning all or part of those separatedmicrobial cells back to the fermentor. Generally, a filtration device isused to accomplish separations.

The term “fermentor”, “reactor vessel” or “bioreactor”, includes afermentation device consisting of one or more vessels and/or towers orpiping arrangements, which includes the Continuous Stirred Tank Reactor(CSTR), Immobilized Cell Reactor (ICR), Trickle Bed Reactor (TBR),Moving Bed Biofilm Reactor (MBBR), Bubble Column, Gas Lift Fermenter,Membrane Reactor such as Hollow Fibre Membrane Bioreactor (HFMBR),Static Mixer, or other vessel or other device suitable for gas-liquidcontact.

CO-Containing Substrate

A CO-containing substrate may include any gas that includes CO. In thisaspect, a CO-containing gas may include syngas, industrial gases, andmixtures thereof.

Syngas may be provided from any know source. In one aspect, syngas maybe sourced from gasification of carbonaceous materials. Gasificationinvolves partial combustion of biomass in a restricted supply of oxygen.The resultant gas mainly includes CO and H₂. In this aspect, syngas willcontain at least about 10 mole % CO, in one aspect, at least about 20mole %, in one aspect, about 10 to about 100 mole %, in another aspect,about 20 to about 100 mole % CO, in another aspect, about 30 to about 90mole % CO, in another aspect, about 40 to about 80 mole % CO, and inanother aspect, about 50 to about 70 mole % CO. Some examples ofsuitable gasification methods and apparatus are provided in U.S. Ser.Nos. 61/516,667, 61/516,704 and 61/516,646, all of which were filed onApr. 6, 2011, and in U.S. Ser. Nos. 13/427,144, 13/427,193 and13/427,247, all of which were filed on Mar. 22, 2012, and all of whichare incorporated herein by reference.

In another aspect, the process has applicability to supporting theproduction of alcohol from gaseous substrates such as high volumeCO-containing industrial flue gases. In some aspects, a gas thatincludes CO is derived from carbon containing waste, for example,industrial waste gases or from the gasification of other wastes. Assuch, the processes represent effective processes for capturing carbonthat would otherwise be exhausted into the environment. Examples ofindustrial flue gases include gases produced during ferrous metalproducts manufacturing, non-ferrous products manufacturing, petroleumrefining processes, gasification of coal, gasification of biomass,electric power production, carbon black production, ammonia production,methanol production and coke manufacturing.

Depending on the composition of the CO-containing substrate, theCO-containing substrate may be provided directly to a fermentationprocess or may be further modified to include an appropriate H₂ to COmolar ratio. In one aspect, CO-containing substrate provided to thefermentor has an H₂ to CO molar ratio of about 0.2 or more, in anotheraspect, about 0.25 or more, and in another aspect, about 0.5 or more. Inanother aspect, CO-containing substrate provided to the fermentor mayinclude about 40 mole percent or more CO plus H₂ and about 30 molepercent or less CO, in another aspect, about 50 mole percent or more COplus H₂ and about 35 mole percent or less CO, and in another aspect,about 80 mole percent or more CO plus H₂ and about 20 mole percent orless CO.

In one aspect, the CO-containing substrate mainly includes CO and H₂. Inthis aspect, the CO-containing substrate will contain at least about 10mole % CO, in one aspect, at least about 20 mole %, in one aspect, about10 to about 100 mole %, in another aspect, about 20 to about 100 mole %CO, in another aspect, about 30 to about 90 mole % CO, in anotheraspect, about 40 to about 80 mole % CO, and in another aspect, about 50to about 70 mole % CO. The CO-containing substrate will have a CO/CO₂ratio of at least about 0.75, in another aspect, at least about 1.0, andin another aspect, at least about 1.5.

In one aspect, a gas separator is configured to substantially separateat least one portion of the gas stream, wherein the portion includes oneor more components. For example, the gas separator may separate CO₂ froma gas stream comprising the following components: CO, CO₂, H₂, whereinthe CO₂ may be passed to a CO₂ remover and the remainder of the gasstream (comprising CO and H₂) may be passed to a bioreactor. Any gasseparator known in the art may be utilized. In this aspect, syngasprovided to the fermentor will have about 10 mole % or less CO₂, inanother aspect, about 1 mole % or less CO₂, and in another aspect, about0.1 mole % or less CO₂.

Certain gas streams may include a high concentration of CO and lowconcentrations of H₂. In one aspect, it may be desirable to optimize thecomposition of the substrate stream in order to achieve higherefficiency of alcohol production and/or overall carbon capture. Forexample, the concentration of H₂ in the substrate stream may beincreased before the stream is passed to the bioreactor.

According to particular aspects of the invention, streams from two ormore sources can be combined and/or blended to produce a desirableand/or optimized substrate stream. For example, a stream comprising ahigh concentration of CO, such as the exhaust from a steel millconverter, can be combined with a stream comprising high concentrationsof H₂, such as the off-gas from a steel mill coke oven.

Depending on the composition of the gaseous CO-containing substrate, itmay also be desirable to treat it to remove any undesired impurities,such as dust particles before introducing it to the fermentation. Forexample, the gaseous substrate may be filtered or scrubbed using knownmethods.

Bioreactor Design and Operation

Descriptions of fermentor designs are described in U.S. Ser. Nos.13/471,827 and 13/471,858, both filed May 15, 2012, and U.S. Ser. No.13/473,167, filed May 16, 2012, all of which are incorporated herein byreference.

In accordance with one aspect, the fermentation process is started byaddition of medium to the reactor vessel. Some examples of mediumcompositions are described in U.S. Ser. Nos. 61/650,098 and 61/650,093,filed May 22, 2012, and in U.S. Pat. No. 7,285,402, filed Jul. 23, 2001,all of which are incorporated herein by reference. The medium may besterilized to remove undesirable microorganisms and the reactor isinoculated with the desired microorganisms. Sterilization may not alwaysbe required.

In one aspect, the microorganisms utilized include acetogenic bacteria.Examples of useful acetogenic bacteria include those of the genusClostridium, such as strains of Clostridium ljungdahlii, including thosedescribed in WO 2000/68407, EP 117309, U.S. Pat. Nos. 5,173,429,5,593,886 and 6,368,819, WO 1998/00558 and WO 2002/08438, strains ofClostridium autoethanogenum (DSM 10061 and DSM 19630 of DSMZ, Germany)including those described in WO 2007/117157 and WO 2009/151342 andClostridium ragsdalei (P11, ATCC BAA-622) and Alkalibaculum bacchi(CP11, ATCC BAA-1772) including those described respectively in U.S.Pat. No. 7,704,723 and “Biofuels and Bioproducts from Biomass-GeneratedSynthesis Gas”, Hasan Atiyeh, presented in Oklahoma EPSCoR Annual StateConference, Apr. 29, 2010 and Clostridium carboxidivorans (ATCCPTA-7827) described in U.S. Patent Application No. 2007/0276447. Othersuitable microorganisms includes those of the genus Moorella, includingMoorella sp. HUC22-1, and those of the genus Carboxydothermus. Each ofthese references is incorporated herein by reference. Mixed cultures oftwo or more microorganisms may be used.

Some examples of useful bacteria include Acetogenium kivui,Acetoanaerobium noterae, Acetobacterium woodii, Alkalibaculum bacchiCP11 (ATCC BAA-1772), Blautia producta, Butyribacteriummethylotrophicum, Caldanaerobacter subterraneous, Caldanaerobactersubterraneous pacificus, Carboxydothermus hydrogenoformans, Clostridiumaceticum, Clostridium acetobutylicum, Clostridium acetobutylicum P262(DSM 19630 of DSMZ Germany), Clostridium autoethanogenum (DSM 19630 ofDSMZ Germany), Clostridium autoethanogenum (DSM 10061 of DSMZ Germany),Clostridium autoethanogenum (DSM 23693 of DSMZ Germany), Clostridiumautoethanogenum (DSM 24138 of DSMZ Germany), Clostridium carboxidivoransP7 (ATCC PTA-7827), Clostridium coskatii (ATCC PTA-10522), Clostridiumdrakei, Clostridium ljungdahlii PETC (ATCC 49587), Clostridiumljungdahlii ERI2 (ATCC 55380), Clostridium ljungdahlii C-01 (ATCC55988), Clostridium ljungdahlii O-52 (ATCC 55889), Clostridium magnum,Clostridium pasteurianum (DSM 525 of DSMZ Germany), Clostridium ragsdaliP11 (ATCC BAA-622), Clostridium scatologenes, Clostridiumthermoaceticum, Clostridium ultunense, Desulfotomaculum kuznetsovii,Eubacterium limosum, Geobacter sulfurreducens, Methanosarcinaacetivorans, Methanosarcina barkeri, Morrella thermoacetica, Morrellathermoautotrophica, Oxobacter pfennigii, Peptostreptococcus productus,Ruminococcus productus, Thermoanaerobacter kivui, and mixtures thereof.

The fermentation should desirably be carried out under appropriateconditions for the desired fermentation to occur (e.g. CO-to-ethanol).Reaction conditions that should be considered include pressure,temperature, gas flow rate, liquid flow rate, media pH, media redoxpotential, agitation rate (if using a continuous stirred tank reactor),inoculum level, maximum gas substrate concentrations to ensure that COin the liquid phase does not become limiting, and maximum productconcentrations to avoid product inhibition.

The methods of the invention can be used to sustain the viability of amicrobial culture, wherein the microbial culture is limited in CO, suchthat the rate of transfer of CO into solution is less than the uptakerate of the culture. Such situations may arise when a substratecomprising CO is not continuously provided to the microbial culture; themass transfer rate is low; or there is insufficient CO in a substratestream to sustain culture vitality at optimum temperature. In suchembodiments, the microbial culture will rapidly deplete the CO dissolvedin the liquid nutrient medium and become substrate limited as furthersubstrate cannot be provided fast enough.

Startup:

Upon inoculation, an initial feed gas supply rate is establishedeffective for supplying the initial population of microorganisms.Effluent gas is analyzed to determine the content of the effluent gas.Results of gas analysis are used to control feed gas rates. In thisaspect, the process may provide a calculated CO concentration to initialcell density ratio of about 0.5 to about 0.9, in another aspect, about0.6 to about 0.8, in another aspect, about 0.5 to about 0.7, and inanother aspect, about 0.5 to about 0.6.

In another aspect, a fermentation process includes providing syngas to afermentation medium in an amount effective for providing an initialcalculated CO concentration in the fermentation medium of about 0.15 mMto about 0.70 mM, in another aspect, about 0.15 mM to about 0.50 mM, inanother aspect, about 0.15 mM to about 0.35 mM, in another aspect, about0.20 mM to about 0.30 mM, and in another aspect, about 0.23 mM to about0.27 mM. The process is effective for increasing cell density ascompared to a starting cell density.

In one aspect, a process for fermenting a CO-containing gaseoussubstrate includes providing a CO-containing gaseous substrate to afermentation medium and maintaining a conductivity to specific carbonuptake (SCU in mmole/minute/gram dry cells) according to a formula whereSCU=SCU_(max)−F*conductivity, wherein SCU_(max)=0 to 3 and F=0 to 1.FIG. 31 graphically illustrates this equation. In this aspect, thefermentation medium has a conductivity of about 30 mS/cm or less and inother aspects, may have the conductivity as describe herein. In anotheraspect, F (which is the slope of the line) may be 0 to 1, in anotheraspect, 0.05 to 1, in another aspect, 0.1 to 1, in another aspect, 0.2to 1, in another aspect, 0.3 to 1, in another aspect, 0.4 to 1, and inanother aspect, 0.5 to 1.

In one aspect, the process includes maintaining a conductivity (y) tospecific gas feed rate (x) according to a formula y=−10.109x+14.2 untilreaching a target cell density. FIG. 30 graphically illustrates thisequation. In this aspect, the fermentation medium has a conductivity ofabout 30 mS/cm or less and in other aspects, may have the conductivityas describe herein. In this aspect, x is about 0.2 to about 0.7mmole/minute/gram of cells. In another aspect, x is about 0.3 to about0.6 mmole/minute/gram of cells, and in another aspect, x is about 0.4 toabout 0.5 mmole/minute/gram of cells.

In another aspect, the process is effective for providing a target celldensity of about 3 to about 30 g/L, in another aspect, about 4 to about25 g/L, in another aspect, about 5 to about 25 g/L, in another aspect,about 7 to about 25 g/L, in another aspect, about 10 to about 25 g/L, inanother aspect, about 12 to about 20 g/L, and in another aspect, about15 to about 20 g/L.

Post-Startup:

Upon reaching desired levels, liquid phase and cellular material iswithdrawn from the reactor and replenished with medium. The process iseffective for increasing cell density to about 2.0 grams/liter or more,in another aspect, about 2 to about 30 grams/liter, in another aspect,about 2 to about 25 grams/liter, in another aspect, about 2 to about 20grams/liter, in another aspect, about 2 to about 10 grams/liter, inanother aspect, about 2 to about 8 grams/liter, in another aspect, about3 to about 30 grams/liter, in another aspect, about 3 to about 6grams/liter, and in another aspect, about 4 to about 5 grams/liter.

Upon reaching a target cell density, the process is effective formaintaining a cell density. Cell density may be maintained through cellrecycle. The process may utilize cell recycle to increase or decreasecell concentration inside the reactor. In this aspect, liquid effluentfrom the reactor is sent to a cell separator where cells and permeateare separated. Cells may be sent back to the reactor. Cell density maybe controlled through a recycle filter. Some examples of bioreactors andcell recycle are described U.S. Ser. Nos. 61/571,654 and 61/571,565,filed Jun. 30, 2011, U.S. Ser. No. 61/573,845, filed Sep. 13, 2011, U.S.Ser. Nos. 13/471,827 and 13/471,858, filed May 15, 2012, and U.S. Ser.No. 13/473,167, filed May 16, 2012, all of which are incorporated hereinby reference.

In one aspect, the process is effective for maintaining an H₂ conversionof about 25% or more. In another aspect, the process is effective formaintaining an H₂ conversion of about 25% to about 95%, in anotheraspect, about 30% to about 90%, in another aspect, about 35% to about85%, in another aspect, about 40% to about 80%, in another aspect, about40% to about 70%, in another aspect, about 40% to about 60%, and inanother aspect, about 40% to about 50%.

In another aspect, the process is effective for maintaining a CO uptakein a range of about 0.001 to about 10 mmole/minute/gram of dry cells. Inanother aspect, the process is effective for maintaining CO uptake in arange of about 0.001 to about 5 mmole/minute/gram of dry cells, inanother aspect, about 0.001 to about 4 mmole/minute/gram of dry cells,in another aspect, about 0.001 to about 3 mmole/minute/gram of drycells, in another aspect, about 0.001 to about 2 mmole/minute/gram ofdry cells, in another aspect, about 0.001 to about 1 mmole/minute/gramof dry cells, in another aspect, about 0.05 to about 9 mmole/minute/gramof dry cells, in another aspect, about 0.05 to about 5 mmole/minute/gramof dry cells, in another aspect, about 0.05 to about 4 mmole/minute/gramof dry cells, in another aspect, about 0.05 to about 3 mmole/minute/gramof dry cells, in another aspect, about 0.05 to about 2 mmole/minute/gramof dry cells, in another aspect, about 0.05 to about 1 mmole/minute/gramof dry cells, in another aspect, about 1 to about 8 mmole/minute/gram ofdry cells, in another aspect, about 1 to about 5 mmole/minute/gram ofdry cells, in another aspect, about 1 to about 4 mmole/minute/gram ofdry cells, in another aspect, about 1 to about 3 mmole/minute/gram ofdry cells, and in another aspect, about 1 to about 2 mmole/minute/gramof dry cells.

In one aspect, the process is effective for providing a CO conversion ofabout 5 to about 99%. In another aspect, CO conversion is about 10 toabout 90%, in another aspect, about 20 to about 80%, in another aspect,about 30 to about 70%, in another aspect, about 40 to about 60%, inanother aspect, about 50 to about 95%, in another aspect, about 60 toabout 95%, in another aspect, about 70 to about 95%, and in anotheraspect, about 80 to about 95%.

Control of Medium Conductivity

Use of mediums formulated to have lower conductivity and/or adjustmentof medium conductivity by dilution are effective for controlling mediumconductivity. In one aspect, the process is effective for providing anaverage conductivity of about 30 mS/cm or less, in another aspect, about25 mS/cm or less, in another aspect, about 20 mS/cm or less, in anotheraspect, 16 mS/cm or less, in another aspect, about 12 mS/cm or less, inanother aspect, about 8 mS/cm or less, in another aspect, about 6.5mS/cm or less, in another aspect, about 6.0 mS/cm or less, in anotheraspect, about 5.5 mS/cm or less, in another aspect, about 5.0 mS/cm orless, in another aspect, about 4.7 mS/cm or less, in another aspect,about 4.5 mS/cm or less, in another aspect, about 4.0 mS/cm to about 6.5mS/cm, in another aspect, about 5.0 mS/cm to about 6.0 mS/cm, and inanother aspect, about 4.0 mS/cm to about 5.0 mS/cm.

In accordance with one aspect, the fermentation process is started byaddition of a suitable medium to the reactor vessel. The liquidcontained in the reactor vessel may include any type of suitablenutrient medium or fermentation medium. The nutrient medium will includevitamins and minerals effective for permitting growth of themicroorganism being used. Anaerobic medium suitable for the fermentationof ethanol using CO as a carbon source are known. One example of asuitable fermentation medium is described in U.S. Pat. No. 7,285,402,which is incorporated herein by reference. Other examples of suitablemedium are described in U.S. Ser. Nos. 61/650,098 and 61/650,093, bothfiled on May 22, 2012, and which are both incorporated herein byreference. In one aspect, the medium utilized includes less than about0.01 g/L yeast extract and less than about 0.01 g/L carbohydrates.

Substitution of Chloride Ion:

In one aspect, the process provides mediums having an averageconductivity of less than about 30 mS/cm by substituting chloride ionsin the medium with a non-chloride ion. More specifically, ammoniumchloride may be substituted with a nitrogen source selected from thegroup consisting of ammonium hydroxide, ammonium acetate, ammoniumcarbonate, ammonium bicarbonate and mixtures thereof.

In one aspect, the medium includes at least one or more of a nitrogensource, at least one or more phosphorous source and at least one or moreof a potassium source. The medium may include any one of the three, anycombination of the three, and in an important aspect, includes allthree. A phosphorous source may include a phosphorous source selectedfrom the group consisting of phosphoric acid, ammonium phosphate,potassium phosphate, and mixtures thereof. A potassium source mayinclude a potassium source selected from the group consisting ofpotassium chloride, potassium phosphate, potassium nitrate, potassiumsulfate, and mixtures thereof.

In one aspect, the medium includes one or more of iron, tungsten,nickel, cobalt, magnesium, sulfur and thiamine. The medium may includeany one of these components, any combination, and in an importantaspect, includes all of these components. An iron may include an ironsource selected from the group consisting of ferrous chloride, ferroussulfate, and mixtures thereof. A tungsten source may include a tungstensource selected from the group consisting of sodium tungstate, calciumtungstate, potassium tungstate, and mixtures thereof. A nickel sourcemay include a nickel source selected from the group consisting of nickelchloride, nickel sulfate, nickel nitrate, and mixtures thereof. A cobaltsource may include a cobalt source selected from the group consisting ofcobalt chloride, cobalt fluoride, cobalt bromide, cobalt iodide andmixtures thereof. A magnesium source may include a magnesium sourceselected from the group consisting of magnesium chloride, magnesiumsulfate, magnesium phosphate, and mixtures thereof. A sulfur source mayinclude cysteine, sodium sulfide, and mixtures thereof.

Concentrations of various components are as follows:

Concentration Range Preferred Range (expressed as mg (expressed as mg orμg nutrient per or μg nutrient per Component gram of cells) gram ofcells) nitrogen (N) 100-340 mg 190-210 mg phosphorus (P) 10.5-15 mg12-13 mg potassium (K) 26-36 mg 28-33 mg iron (Fe) 2.7-5 mg 3.0-4.0 mgtungsten (W) 10-30 μg 15-25 μg Nickel (Ni) 34-40 μg 35-37 μg Cobalt (Co)9-30 μg 15-20 μg Magnesium (Mg) 4.5-10 mg 5-7 mg Sulfur (S) 11-20 mg12-16 mg Thiamine 6.5-20 μg 7-12 μg

Process operation maintains a pH in a range of about 4.2 to about 4.8.The medium includes less than about 0.01 g/L yeast extract and less thanabout 0.01 g/L carbohydrates.

In another aspect, the process control medium conductivity throughdilution of medium. In this aspect, once the fermentation reaches aconductivity of about 30 mS/cm, the process includes addition of wateror a low conductivity medium to the fermentation in an amount effectiveto lower the medium conductivity.

EXAMPLES Example 1: Effect of Conductivity on Growth

Clostridium ljungdahlii was grown in a bioreactor (New Brunswick BioFloI or IIc). The following adjustments were made:

Conductivity of the culture was adjusted by adjusting the strength ofthe growth medium, for example concentration of all the components,except vitamin in the growth medium was increased by 1.5 times toincrease the conductivity of the culture from approximately 7 mS toapproximately 9.5 mS.

All experiments were started with the initial cell density of 0.38(+/−0.02) or 0.48 g/L.

Initial gas flow rate of each experiment was kept unchanged throughoutthe experiment. Reactor parameters, when CO conversion values reach aplateau after a successful start-up, were used to calculate K_(La), forrelevant conditions.

Syngas composition was 30% CO, 15% H₂, 10% CO₂ and 45% N₂.

Bioreactor Run #1:

1× growth medium and 25 ml/min syngas feed rate was used in thisexperiment. As shown in FIG. 1, after an initial lag period of about 20hours bacteria started to multiply at a doubling time of about 20 hours.Maximum calculated dissolved CO was about 0.22 mmol in the reactorbroth. (D CO: dissolved CO concentration in the reactor broth, CD: celldensity, SCU specific CO uptake.)

Bioreactor Run #2:

1× growth medium and 35 ml/min syngas feed rate was used in thisexperiment. As shown in FIG. 2, after initial lag period of about 36hours bacteria started to multiply at a doubling time of about 20 hours.Maximum calculated dissolved CO was about 0.22 mmol in the reactorbroth.

Bioreactor Run #3:

1× growth medium and 40 ml/min syngas feed rate was used in thisexperiment. As shown in the FIG. 3, after initial lag period of about 45hours bacteria started to multiply at a doubling time of about 20 hours.Maximum calculated dissolved CO was about 0.22 mmol in the reactorbroth.

Bioreactor Run #4:

1× growth medium and 45 ml/min syngas feed rate was used in thisexperiment. As shown in the FIG. 4, after an initial lag period of about50 hours, bacteria started to multiply at a doubling time of about 20hours. Maximum calculated dissolved CO was around 0.17 mmol in thereactor broth.

Bioreactor Run #5:

1× growth medium and 50 ml/min syngas feed rate was used in thisexperiment. As shown in FIG. 5, culture continued to lag even at about70 hours after inoculation. Maximum calculated dissolved CO was about0.23 mmol in the reactor broth.

Bioreactor Run #6:

1× growth medium and 50 ml/min syngas feed rate was used in thisexperiment. This experiment was started with an inoculum of 4.8 g/L ofbacteria. As shown in FIG. 6, after an initial lag period of about 10hours, bacteria started to multiply at a doubling time of about 20hours. Maximum calculated dissolved CO was about 0.12 mmol in thereactor broth.

Bioreactor Run #7:

1.5× growth medium and 45 ml/min syngas feed rate was used in thisexperiment. This experiment was started with an inoculum of 3.8 g/L ofbacteria. As shown in FIG. 7, bacterial cell density went down withtime. Maximum calculated dissolved CO was about 0.25 mmol in the reactorbroth.

Bioreactor Run #8:

1.5× growth medium and 35 ml/min syngas feed rate was used in thisexperiment. This experiment was started with an inoculum of 3.8 g/L ofbacteria. As shown in FIG. 8, bacterial cell density went down withtime. Maximum calculated dissolved CO was about 0.22 mmol in the reactorbroth.

Bioreactor Run #9:

1.5× growth medium and 30 ml/min syngas feed rate was used in thisexperiment. This experiment was started with an inoculum of 3.8 g/L ofbacteria. As shown in FIG. 9, bacterial cell density went down withtime. Maximum calculated dissolved CO was around 0.22 mmol in thereactor broth.

Bioreactor Run #10:

1.5× growth medium and 20 ml/min syngas feed rate was used in thisexperiment. This experiment was started with an inoculum of 3.8 g/L ofbacteria. As shown in FIG. 10, bacterial cell density went up with timeand achieved a doubling time of about 20 hours. Maximum calculateddissolved CO was around 0.22 mmol in the reactor broth.

Example 2: Growth on Alternative Nitrogen Sources

Clostridium ljungdahlii C-01 was grown in a bioreactor (BioFlo/CelliGen115) with the following medium.

Chemical Target FeCl₂*4H₂O (g) 0.24 H₃PO₄ (ml) 0.86 KCl (g) 3.00MgCl₂*6H₂O (g) 0.48 NH₄Cl (g) 19.44 Cysteine HCl (g) 4.50 6x Med A2 (ml)15.0 6x TE (ml) 4.6 Water (L) to 10

For each experiment, the NH₄Cl was omitted from the medium and replacedwith molar equivalents of one of the nitrogen containing compoundsdescribes below.

Chemical Target (g) l-Lysine 26.57 Ammonium acetate 28.01 NH₄C₂H₃O₂Ammonium carbonate 17.46 (NH₄)₂CO₃ Ammonium bicarbonate 28.73 (NH₄)HCO₃

The pH of these media was adjusted to ˜4.0-4.4. Ammonium carbonate wasalso tested as base solution by using both 0.25M (24.02 g/L) and 0.125M(12.01 g/L) concentrations as substitute for 7.7% NaHCO₃.

Specific carbon uptake of Clostridium ljungdahlii growing in mediumcontaining ammonium chloride is shown in FIG. 11.

Specific ethanol productivity of Clostridium ljungdahlii growing inmedium containing ammonium chloride is shown in FIG. 12.

Specific carbon uptake of Clostridium ljungdahlii growing in mediumcontaining 1-Lysine is shown in FIG. 13.

Specific ethanol productivity of Clostridium ljungdahlii growing inmedium containing 1-Lysine is shown in FIG. 14.

Conductivity of Clostridium ljungdahlii growing in medium containing1-Lysine is shown in FIG. 15.

Specific carbon uptake of Clostridium ljungdahlii growing in mediumcontaining ammonium acetate is shown in FIG. 16.

Specific ethanol productivity of Clostridium ljungdahlii growing inmedium containing ammonium acetate is shown in FIG. 17.

Conductivity of Clostridium ljungdahlii growing in medium containingammonium acetate is shown in FIG. 18.

Specific carbon uptake of Clostridium ljungdahlii growing in mediumcontaining ammonium carbonate is shown in FIG. 19.

Specific ethanol productivity of CL growing in medium containingammonium carbonate is shown in FIG. 20.

Conductivity of Clostridium ljungdahlii growing in medium containingammonium carbonate is shown in FIG. 21.

Specific carbon uptake of Clostridium ljungdahlii growing in medium withammonium carbonate as base is shown in FIG. 22.

Specific ethanol productivity of CL growing in medium with ammoniumcarbonate as base is shown in FIG. 23.

Conductivity of Clostridium ljungdahlii growing in medium with ammoniumcarbonate as base is shown in FIG. 24.

Specific carbon uptake of Clostridium ljungdahlii growing in medium withammonium bicarbonate is shown in FIG. 25.

Specific ethanol productivity of Clostridium ljungdahlii growing inmedium with ammonium bicarbonate is shown in FIG. 26.

Conductivity of Clostridium ljungdahlii growing in medium with ammoniumbicarbonate is shown in FIG. 27.

The following Tables summarize the results.

Specific Carbon Uptake and Specific Ethanol Productivity

SCU SCU Sp. EtOH Productivity Treatment Average SD Average Baseline0.711 0.054 5.706 l-Lysine 0.521 0.179 6.421 (medium) Ammonium acetate0.792 0.082 7.862 (medium) Ammonium carbonate 0.846 0.075 9.009 (medium)Ammonium carbonate 0.883 0.039 9.718 (base) Total  0.25M 0.867 0.0349.091 0.125M 0.894 0.038 10.165 Ammonium bicarbonate 0.848 0.047 8.726(medium)

Conductivity of Fermentor Medium (mS/cm)

Conductivity Conductivity Treatment Raw Decrease* Average l-Lysine −5.773.291 (medium) Ammonium acetate −4.75 4.027 (medium) Ammonium carbonate−3.74 4.839 (medium) Ammonium carbonate −6.30 2.445 (base) Total  0.25M−5.73 3.065 0.125M −6.30 1.982 Ammonium bicarbonate −4.05** 4.300(medium) *Uses the initial value from the Lysine experiment (8.07 mS/cm)as the baseline value. **Omits outlying measurements near beginning ofexperiment.

Ammonium Ion Concentration

Ammonium Ion Concentration (ppm) Treatment Average Baseline 145.71Ammonium carbonate 235.26 (0.25M base) Ammonium carbonate 220.11 (0.125Mbase) Ammonium bicarbonate 184.41 (medium)

Results indicate that nitrogenous compounds, especially those containingammonium ions, can be used as substitutes for ammonium chloride.L-Lysine used in medium was not successful as a nitrogen source toobtain high performance. Lysine use led to initial gains in bothspecific carbon uptake (SCU) and specific ethanol productivity (SEP),but ultimately led to marked decreases in both of those metrics. The rawvalue for SCU was decreased by 58%, while the raw value for SEP wasdecreased by 47%. Average SCU was decreased by 26%, a significantchange. Average SEP was increased by 12%, but that increase was notstatistically significant due to a very large standard deviation. Celldensity decreased over the course of the experiment from 2.03 g/l to1.02 g/l. Conductivity raw value decreased 71%, from 8.07 mS/cm to 2.30mS/cm, with an average over the time course of the experiment of 3.291mS/cm. The majority of the time in this experiment, conductivity wasless than 3.0 mS/cm.

Ammonium acetate as nitrogen source in medium led to a slight increaseof only 11% in average SCU, a value which is not statisticallysignificant. However, the raw value for SCU decreased from the beginningto the end of the experiment (0.943 vs. 0.830), a 12% drop. There was asignificant increase of 37% in average SEP when compared to baselineaverages. The raw value for SEP increased during the experiment from6.43 to 8.57, an increase of 25%. Conductivity during this experimentdecreased by 58%, to a low of 3.32 mS/cm (using the initial value fromthe lysine experiment as a baseline for conductivity in PP-Al medium),with an average value of 4.027 mS/cm. For the majority of the experimentthe conductivity value was less than 4.0.

When ammonium carbonate was used in the medium as a nitrogen source,significant increases in both SCU and SEP were observed. SCU increasedby 19% and SEP increased by 57%. The raw values for SCU and SEPdecreased during the experiment by 15% and 23%, respectively.Conductivity during this experiment decreased by 46%, to a low of 4.33mS/cm, with an average value of 4.839 mS/cm.

Ammonium carbonate was also tested as nitrogen source by omitting thecompound from the medium recipe and instead using it as the basesolution. This method of supplying ammonium carbonate resulted inoverall significant increases in SCU (19%) and SEP (41%) versusbaseline. Two different concentrations of ammonium carbonate were used,0.25M and 0.125M, and there were slightly different results for each.Each concentration yielded significantly higher SCU and SEP than thereactor baseline. Each base concentration resulted in slightly differentvalues for the two measured metrics, but the standard deviations ofthese measurements overlap each other. The conductivity was decreased bythe two base solutions by 71% (0.25M) and 78% (0.125M) to respectivelows of 2.34 mS/cm and 1.77 mS/cm. The averages were 3.065 mS/cm and1.982 mS/cm, respectively. The concentration of the ammonium ion in thereactor was measured just before and throughout the duration of theammonium carbonate as base experiment. The results of these measurementsshow that the reactor was supplied with excess ammonium ion (50-62%)during the experiment.

Ammonium bicarbonate was also tested as an additive to the medium. Theexperimental data show that there were significant increases in both SCUand SEP. Average SCU was increased above the baseline by 19% and averageSEP was increased above baseline by 53%. Conductivity during thisexperiment decreased by 50%, to a low of 4.02 mS/cm, with an averagevalue of 4.3 mS/cm. Ammonium ion concentration was also monitored duringthis phase of the experiment. The values show that the ion was in excessby 26% in the reactor.

Example 3: Effects of Step-Wise Increases in Osmolarity on CulturePerformance

Clostridium ljungdahlii C-01 was grown in a bioreactor (BioFlo/CelliGen115) with the following medium. The average media flow per gram of cellswas 1 mL/g/minute.

Chemical Target FeCl₂*4H₂O (g) 0.24 H₃PO₄ (ml) 0.86 KCl (g) 3.00MgCl₂*6H₂O (g) 0.48 NH₄Cl (g) 19.44 Cysteine HCl (g) 4.50 6x Med A2 (ml)15.0 6x TE (ml) 4.6 Water (L) to 10

In this 21 day experiment the conductivity of the culture broth wasincreased using NaCl. Each addition of NaCl at given intervals are shownin FIG. 28 according to the following schedule.

NaCl Concentrations after Each Addition.

Time (hrs) Concentration (g/L) 163 1 307 2 422 3 498 4 597 5.23 618 5646 6 720 7 793 8 883 9 935 10 1001 11 1057 12 1119 13

The conductivity of the culture rose with each addition of NaCl.Specific Carbon Uptake (SCU), an indicator of culture activity wasmeasured through out the experiment. FIG. 28 shows that with each NaCladdition the SCU was diminished for a period of time but would recoverafter a short adaptation period.

FIG. 28 can be divided into three areas of interest; 0-500 hrs (1),500-1100 hrs (2), and 1100-1200 hrs. Area 1, where the conductivity wasless than 15 mS/cm, shows addition of NaCl has less impact on the SCU:only small losses of SCU followed by full recoveries. Area 2, where theconductivity is above 15 mS/cm, shows addition of NaCl has a higherimpact on the SCU: large swings of SCU. In this area the NaCl additionscaused large drops in SCU followed by large up-swings. In the final areawhen the conductivity rose to about 30 mS/cm or higher culture lost itsactivity.

Example 4: Effects of Rapid Increases in Osmolarity on CulturePerformance

Clostridium ljungdahlii C-01 was grown in a bioreactor (BioFlo/CelliGen115) with the same medium as described in Example 3. The average mediaflow per gram of cells was 1.1 mL/g/minute.

In this 10 day experiment, NaCl concentration in the broth was increasedtwice as fast the rate of increase of NaCl concentration in Example 3according to the following schedule.

NaCl Concentrations after Each Addition.

Time (hrs) Concentration (g/L) 96 7 121 9 144 11 498 13

FIG. 29 shows SCU of the culture at different conductivities. As in theExample 3 culture lost its activity once the conductivity of the culturereach around 30 mS/cm.

Example 5: Effect of CO Feed Rate on Conductivity

Clostridium ljungdahlii C-01 was grown in a bioreactor (BioFlo/CelliGen115) with the same medium as described in Example 3.

Conductivity of the culture was adjusted by adjusting the strength ofthe growth medium, for example concentration of all the components,except vitamin in the growth medium was increased by 1.5 and 2 times toincrease the conductivity of the culture from ˜7 mS to ˜9.5 mS and ˜12mS respectively.

Experiments were started with the initial cell density of 0.38 (+/−0.02)or 0.785 g/l. Syngas composition was 30% CO, 15% H₂, CO₂ 10% and 45% N₂.Several start-up experiments were done at each given cultureconductivity to determine the appropriate (that can be practically used)specific gas feed rate for given culture conductivity. Through theseexperiments the appropriate gas feed rate was determined for a givenculture conductivity. As illustrated in FIG. 30, theappropriate/functional CO feed rate was plotted against cultureconductivity, where

y=−6.0327x+12.901

Specific CO feed rate=molar amounts of CO per gram of cells

Appropriate/functional CO feed rate=the CO feed rate that C-01 candouble within 40 hours.

While the invention herein disclosed has been described by means ofspecific embodiments, examples and applications thereof, numerousmodifications and variations could be made thereto by those skilled inthe art without departing from the scope of the invention set forth inthe claims.

1-14. (canceled)
 15. A process for fermentation of a CO-containinggaseous substrate, the process comprising: introducing the CO-containinggaseous substrate into a reactor vessel that includes a fermentationmedium and fermenting the CO-containing gaseous substrate with one ormore acetogenic bacteria; wherein at least one or more chloride ions inthe fermentation medium are substituted with an ion selected from thegroup consisting of hydroxide, acetate, carbonate, bicarbonate andmixtures thereof in an amount effective for providing a conductivity ofabout 30 mS/cm or less, wherein the process is effective for maintaininga space time yield (STY) of about 10 g ethanol/(L·day) or more.
 16. Thefermentation process of claim 1 or 15 wherein the fermentation mediumincludes a nitrogen source selected from the group consisting ofammonium hydroxide, ammonium acetate, ammonium carbonate, ammoniumbicarbonate and mixtures thereof.
 17. The fermentation process of claim15 wherein the syngas has a CO/CO₂ ratio of about 0.75 or more. 18.(canceled)
 19. The fermentation process of claim 18 wherein theacetogenic bacteria is selected from the group consisting of Acetogeniumkivui, Acetoanaerobium noterae, Acetobacterium woodii, Blautia producta,Butyribacterium methylotrophicum, Caldanaerobacter subterraneous,Caldanmaerobacter subterraneous pacficus, Carboxydothermushydrogenoformans, Clostridium aceticum, Clostridium acetolmutylicum,Clostridium autoethanogenum Clostridium coskatii (ATCC PTA-10522),Clostridium drakei, Clostridium ljungdahlii PETC (ATCC 49587),Clostridium ljungdahlii ERI2 (ATCC 55380), Clostridium ljungdahlii C-01(ATCC 55988), Clostridium ljungdahlii O-52 (ATCC 55889), Clostridiummagnum, Clostridium pasteurianum (DSM 525 of DSMZ Germany), Clostridiumragsdali P11 (ATCC BAA-622), Clostridium scatologenes, Clostridiumthermoaceticum, Clostridium ultunense, Desulfotomaculum kuznetsovii,Eubacterium limosum, Geobacter sulfurreducens, Methanosarcinaacetivorans, Methanosarcina barkeri, Morrella thermoacetica, Morrellathermoautotrophica, Oxobacter pfennigii, Peptostreptococcus productus,Rutninococcus productus, and Thermoanaerobacter kivuir.
 20. Thefermentation process of claim 15 wherein the process is effective forproviding a cell density of about 2.0 g/L or more.
 21. The fermentationprocess of claim 15 wherein the process is effective for providing a COconversion of about 5 to about 99%.